Microstructure array, and a microlens array

ABSTRACT

In a method of fabricating an array of microstructures, a substrate with an electrically-conductive portion is provided, an insulating mask layer is formed on the electrically-conductive portion of the substrate, a plurality of openings are formed in the insulating mask layer to expose the electrically-conductive portion, and a first plated or electrodeposited layer is deposited in the openings and on the insulating mask layer by electroplating or electrodeposition. A second plated layer is further formed on the first plated or electrodeposited layer and on the electrically-conductive portion by electroless plating to reduce a size distribution of microstructures over the array.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for fabricating amicrostructure array, a method for fabricating a mold or a master of amold (in the specification the term “mold” is chiefly used in a broadsense including both a mold and a master of a mold) for forming amicrostructure array, a method for fabricating a microstructure arrayusing the mold, and a microstructure array. This invention particularlyrelates to a mold for forming a microlens array, a method forfabricating the mold, and a method for fabricating the microlens arrayusing the mold.

[0003] 2. Description of the Related Background Art

[0004] A microlens array typically has a structure of arrayed minutelenses each having a diameter from about 2 or 3 microns to about 200 or300 microns and an approximately semispherical profile. The microlensarray is usable in a variety of applications, such as liquid-crystaldisplay devices, optical receivers and inter-fiber connections inoptical communcation systems.

[0005] Meanwhile, earnest developments have been made with respect to asurface emitting laser and the like which can be readily arranged in anarray form at narrow pitches between the devices. Accordingly, thereexists a significant need for a microlens array with narrow lensintervals and a large numerical aperture (NA).

[0006] Likewise, a light receiving device, such as a charge coupleddevice (CCD), has been repeatedly decreased in size as semiconductorprocessing techniques have been developed and advanced. Therefore, alsoin this field, the need for a microlens array with narrow lens intervalsand a large NA is increasing.

[0007] In the field of such a microlens, a desirable structure is amicrolens with a large light-condensing efficiency which can highlyefficiently utilize light incident on its lens surface.

[0008] Further, similar desires exist in the fields of opticalinformation processing, such as optical parallel processing-operations,and optical interconnections. Furthermore, active or self-radiating typedisplay devices, such as electroluminescence (EL) panels, have beenenthusiastically studied and developed, and a highly-defined andhighly-luminous display has been proposed. In such a display, there is aheightened desire for a microlens array which can be produced at arelatively low cost and with a large area, as well as with a small lenssize and a large NA.

[0009] There are presently a number of prior art methods for fabricatingmicrolenses.

[0010] In a prior art microlens-array fabrication method using an ionexchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1)L51-54, 1981), a refractive index is raised at plural places in asubstrate of multi-component glass. A plurality of lenses are thusformed places with at high-refractive index. In this method, however,the lens diameter cannot be large, compared with the intervals betweenlenses. Hence, it is difficult to design a lens with a large NA.Further, the fabrication of a large-area microlens array is not easysince a large scale manufacturing apparatus, such as an ion diffusionapparatus, is required to produce such a microlens array. Moreover, anion exchange process is needed for each glass, in contrast with amolding method using a mold. Therefore, variations of lens quality, suchas a focal length, are likely to increase between lots unless themanagement of fabrication conditions in the manufacturing apparatus iscarefully conducted. In addition to the above, the cost of this methodis relatively high, as compared with the method using a mold.

[0011] Further, in the ion exchange method, alkaline ions forion-exchange are indispensable in a glass substrate, and therefore, thematerial of the substrate is limited to alkaline glass. The alkalineglass is, however, unfit for a semiconductor-based device which needs tobe free of alkaline ions. Furthermore, since a thermal expansioncoefficient of the glass substrate greatly differs from that of asubstrate of a light radiating or receiving device, misalignment betweenthe microlens array and the devices is likely to occur due to a misfitbetween their thermal expansion coefficients as an integration densityof the devices increases.

[0012] Moreover, a compressive strain inherently remains on the glasssurface which is processed by the ion exchange method. Accordingly, theglass tends to warp, and hence, a difficulty in joining or bondingbetween the glass and the light radiating or receiving device increasesas the size of the microlens array increases.

[0013] In another prior art microlens-array fabrication method using aresist reflow (or melting) method (see D. Daly, et al., Proc. MicrolensArrays Teddington., p23-34, 1991), resin formed on a substrate iscylindrically patterned using a photolithography process and a microlensarray is fabricated by heating and reflowing the resin. Lenses havingvarious shapes can be fabricated at a low cost by this resist reflowmethod. Further, this method has no problems of thermal expansioncoefficient, warp and so forth, in contrast with the ion exchangemethod.

[0014] In the resist reflow method, however, the profile of themicrolens is strongly dependent on the thickness of resin, wettingconditions between the substrate and resin, and the heating temperature.Therefore, variations between lots are likely to occur while fabricationreproducibility per a single substrate surface is high.

[0015] Further, when adjacent lenses are brought into contact with eachother due to the reflow, a desired lens profile cannot be secured due tothe surface tension. Accordingly, it is difficult to achieve a highlight-condensing efficiency by bringing the adjacent lenses into contactand decreasing an unused area between the lenses. Furthermore, when alens diameter from about 20 or 30 microns to about 200 or 300 microns isdesired, the thickness of deposited resin must be large enough to obtaina spherical surface by the reflow. It is, however, difficult touniformly and thickly deposit the resin material having desired opticalcharacteristics (such as refractive index and optical transmissivity).Thus, it is difficult to produce a microlens with a large curvature anda relatively large diameter.

[0016] In another prior art method, an original plate of a microlens isfabricated, lens material is deposited on the original plate and thedeposited lens material is then separated. The original plate or mold isfabricated by an electron-beam lithography method (see Japanese PatentApplication Laid-Open No. 1 (1989)-261601), or a wet etching method (seeJapanese Patent Application Laid-Open No. 5 (1993)-303009). In thesemethods, the microlens can be reproduced by molding, variations betweenlots are unlikely to occur, and the microlens can be fabricated at a lowcost. Further, the problems of alignment error and warp due to thedifference in the thermal expansion coefficient can be solved, incontrast with the ion exchange method.

[0017] In the electron-beam lithography method, however, anelectron-beam lithographic apparatus is expensive and a large investmentin equipment is needed. Further, it is difficult to fabricate a moldhaving a large area more than 100 cm² (100 cm-square) because theelectron beam impact area is limited.

[0018] Further, in the wet etching method, since an isotropic etchingusing a chemical action is principally employed, an etching of the metalplate into a desired profile cannot be achieved if the composition andcrystalline structure of the metal plate vary even slightly. Inaddition, etching will continue unless the plate is washed immediatelyafter a desired shape is obtained. When a minute microlens is to beformed, a deviation of the shape from the desired one is possible due toetching lasting during a period from the time a desired profile isreached to the time the microlens is reached.

[0019] Further, there also exists a mold fabrication method using anelectroplating technique (see Japanese Patent Application Laid-Open No.6 (1994)-27302). In this method, an insulating film having a conductivelayer formed on one surface thereof and an opening is used, theelectroplating is performed with the conductive layer acting as acathode, and a protruding portion acting as a mother mold for a lens isformed on a surface of the insulating film. The process of fabricatingthe mold by this method is simple, and cost is reduced. Similar suchmethods are also disclosed in Japanese Patent Application Laid-Open No.8 (1996)-258051 and Japanese Patent Publication No. 64 (1989)-10169.

[0020] The problem occurring when a plated layer is formed in an openingby the electroplating technique will be described by reference to FIGS.1A and 1B. FIGS. 1A and 1B illustrate a radius variation or distributionof plated layers 105 formed in a two-dimensional array on a substrate101. In the above fabrication method using electroplating in anelectroplating bath, a distribution or variation of anelecroplating-current density occurs over the substrate 101 due to apattern of the openings (i.e., the electrode pattern) formed in aninsulating mask layer 103 to expose an electrode layer 102. Morespecifically, the electric field is unevenly concentrated (stronger in aperipheral region than in a central region), and the electroplatinggrowth is hence promoted near the periphery of the pattern of thearrayed openings. As a result, there is a distribution or variation ofthe size of semispherical microstructures 105 on the substrate.Therefore, when this substrate is used as a mold for forming a microlensarray, the specifications of respective microlenses vary over the array.

SUMMARY OF THE INVENTION

[0021] An object of the present invention is to provide a method forfabricating a microstructure array (typically a microlens array such asa semispherical microlens array, a flyeye lens and a lenticular lens)with good performance and a reduced size distribution of microstructuresflexibly, readily and stably; a method for fabricating a mold forforming a microstructure array; a fabrication method for amicrostructure array using the mold, and so forth.

[0022] The present invention is generally directed to a method forfabricating an array of microstructures which includes the followingsteps:

[0023] preparing a substrate with an electrically-conductive portion;

[0024] forming an insulating mask layer on the electrically-conductiveportion;

[0025] forming a plurality of openings in the insulating mask layer toexpose the electrically-conductive portion;

[0026] forming a first plated or electrodeposited layer in the openingand on the insulating mask layer by electroplating or electrodeposition;and

[0027] forming a second plated layer on the first plated orelectrodeposited layer and on the electrically-conductive portion byelectroless plating to reduce a size distribution of microstructuresover the array.

[0028] In the above fabrication method, the first plated layer is formedby electroplating, or the first electrodeposited layer is formed byelectrodeposition using an electrodepositable organic compound. Thearray pattern is typically a two-dimensional array pattern which isperiodical in at least a direction, a two-dimensional array patternwhich is periodical in four mutually-orthogonal directions, or aperiodical stripe pattern. In terms of an influence of the pattern ofthe conductive portion exposed through the openings on the distributionof a current density at the time of the electroplating orelectrodeposition, the array pattern may create a current distributionin which the current density is uneven over the array. Typically, theopening has a circular shape and the microstructure is a semisphericalmicrostructure, or the opening has an elongated stripe shape and themicrostructure is a semicylindrical microstructure.

[0029] More specifically, the following constructions are possible basedon the above fundamental construction.

[0030] The second plated layer is formed by electroless plating using anelectroless plating solution containing a reducing agent ofhypophosphite such as sodium hypophosphite. Thereby, corrosionresistance and wear resistance of the microstructure array can beimproved.

[0031] The fabrication method may further include a step of forming athird plated layer on the second plated layer by electroplating, or astep of forming a third plated layer on the second plated layer byelectroless plating which preferably uses an electroless platingsolution containing a reducing agent of hypophosphite. Thereby,corrosion resistance and wear resistance of the microstructure array canbe improved.

[0032] The first plated layers may be continuously formed at their skirtportions. A flyeye lens can be fabricated by using the microstructurearray fabricated by such a method.

[0033] In the fabrication method, the first plated or electrodepositedlayer and the second plated layer (additionally, the third plated layer)may be formed such that a horizontal bottom diameter or width of asemispherical or semicylindrical microstructure is approximately in arange from 1 μm to 200 μm. Such a minute microlens array is stronglydesired with accurate size, good controllability and high stability, andthe fabrication method of this invention can meet this desire.

[0034] In the fabrication method, the first plated or electrodepositedlayer and the second plated layer (additionally, the third plated layer)may be formed such that a distribution of horizontal bottom diameters orwidths of semispherical or semicylindrical microstructures (in thisspecification, the distribution is used as a ratio of a differencebetween a maximum value and a minimum value relative to the minimumvalue concerning the size of microstructures) is approximately less than20%. When the size distribution takes such a value, the microstructurearray, such as a mold for forming a microlens array, is of practicaluse.

[0035] In the fabrication method, the first plated or electrodepositedlayer may be formed such that a ratio of a horizontal bottom diameter orwidth of the first plated or electrodeposited layer relative to ahorizontal bottom diameter or width of a semispherical orsemicylindrical microstructures is approximately less than 0.5. Whensuch a condition is satisfied, a satisfactory size distribution can bereadily achieved. As the thickness of the electroless plated layer in avertical direction increases relative to the entire thickness or radiusin the vertical direction of the microstructure, the size distributionof the microstructures decreases. Therefore, in order to better achievea small distribution, a thickness ratio of the electroless plated layerrelative to the entire microstructure is preferably as large aspossible. On the other hand, the process speed of the electrolessplating is lower than that of the electroplating or electrodeposition.The above ratio is determined considering the above factors.

[0036] In the fabrication method, the first plated or electrodepositedlayer may be formed such that a diameter or width of the first plated orelectrodeposited layer is approximately less than 10 μm. Thereby, themicrostructure array with microstructures having a bottom diameter orwidth approximately in a range from 1 μm to 200 μm can be readilyachieved with a preferable distribution.

[0037] The fabrication method can further include a step of forming amold on the substrate with the first plated or electrodeposited layerand the second plated layer (additionally, the third plated layer) by,for example, electroplating, and a step of separating the mold from thesubstrate. Thereby, a mold for forming a microstructure array such as amicrolens array can be fabricated.

[0038] The fabrication method can further include a step of coating thesubstrate having the first plated or electrodeposited layer and thesecond plated layer (additionally, the third plated layer) with a firstresin, a step of hardening the first resin, a step of separating thefirst resin from the substrate, and a step of coating the hardened firstresin with a second resin having a refractive index different from arefractive index of the first resin. Thereby, a preferable microlensarray can be fabricated.

[0039] The present invention is also directed to a microstructure arrayincluding the following:

[0040] a substrate having an electrically-conductive portion;

[0041] an insulating mask layer formed on the electrically-conductiveportion, in which a plurality of openings are formed to expose theelectrically-conductive portion;

[0042] a first plated or electrodeposited layer formed in the openingand on the insulating mask layer by electroplating or electrodeposition;and

[0043] a second plated layer formed on the first plated orelectrodeposited layer and on the electrically-conductive portion byelectroless plating.

[0044] Also in this microstructure array, the above specific structuresmay be adopted. The microstructure array is typically a mold for forminga microlens array, a lenticular lens or a flyeye lens.

[0045] These advantages and others will be more readily understood inconnection with the following detailed description of the more preferredembodiments in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIGS. 1A and 1B are a cross-sectional view and a plan viewillustrating a conventional microstructure array formed on a substrate,respectively.

[0047]FIGS. 2A to 2D are cross-sectional views illustrating fabricationsteps of a method for fabricating a microlens array mold or the like offirst and second embodiments according to the present invention,respectively.

[0048]FIG. 3 is a view illustrating an electroplating apparatus used inthe present invention.

[0049]FIG. 4 is a view illustrating an electroless plating apparatusused in the present invention.

[0050]FIG. 5 is a view illustrating a principle of forming asemispherical or semicylindrical microstructure by electroplating orelectrodeposition used in the present invention.

[0051]FIGS. 6A to 6E are cross-sectional views illustrating fabricationsteps of a fabrication method of fabricating a microlens array mold orthe like of third and fourth embodiments according to the presentinvention, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] (First Embodiment)

[0053] A first embodiment of a method for fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 2A to 2D.

[0054] Initially, a silicon wafer of 1 inch in diameter is thermallyoxidized using an oxidizing gas, and layers of silicon dioxide with athickness of 1 μm are formed on opposite surfaces of the wafer. Thiswafer is used as a substrate 1 illustrated in FIGS. 2A to 2D. Cr and Auare continuously layered with thicknesses of 10 nm and 200 nm on theabove wafer, respectively, using an electron beam vacuum-evaporationmethod which is a suitable thin-film forming method. An electrode layer2 is thus formed.

[0055] A photoresist is then deposited as an insulating mask layer 3 asillustrated in FIG. 2A. Openings 4 are then formed in the mask layer 3by photolithography using exposure and development. A plurality of theopenings 4 are thus formed in a two-dimensional matrix array of 700×700to expose the electrode layer 2 as illustrated in FIG. 2B. The opening 4has a circular shape and a diameter of 5 μm. Intervals between theadjacent openings 4 are 25 μm.

[0056] Ni (nickel) electroplating is then performed for sixty (60)seconds at a bath temperature of 50° C. and a cathodic current densityof 40 A/dm². The above substrate 1 for electroplating is used as a base7, and the electrode layer 2 is used as the cathode as illustrated inFIG. 3. Ni electroplating bath 20 containing nickel (II) sulfate, nickel(II) chloride, boric acid and brightener is used. Ni plated layer 5 isinitially deposited in the opening 4 and grown therein. The plated layer5 expands onto the mask layer 3. The semispherical or semicylindricalplated layer 5 is deposited until a diameter of its bottom portionreaches 11 μm in a central portion of the array, as illustrated in FIG.2C. In FIG. 3, an external electric power source 9 is connected betweenthe base 7 and an anodic plate 8.

[0057] Ni electroless plating is then performed at a bath temperature of90° C. to form electroless plated layers 6 as illustrated in FIG. 2D. Nielectroless plating solution (S-780 (trade name) produced by NihonKanizen Com.) containing a reducing agent of hypophosphite is used.

[0058] The Ni plated layer 6 obtained by the electroless platingcontains phosphorus. When measured, a diameter of a bottom portion ofthe plated layers 5 and 6 in the central portion of the array was 21 μmand a diameter ratio of the electroplated layer 5 relative to the platedlayers 5 and 6 was 0.52. At this time, a diameter of a bottom portion ofthe plated layers 5 and 6 in a peripheral portion of the array was 27 μm(the maximum value). A distribution of diameters of bottoms of theplated layers in central and peripheral portions of the array (i.e., aratio of a difference between maximum and minimum diameters relative tothe minimum diameter) was about 28%.

[0059] For comparison with the above, the above Ni electroless platingwas conducted from beginning to end until the diamter of a bottomportion of a plated layer in a central portion of the array reached 21μm. When diameters of bottoms of the plated layers in several regions ofthe array were measured, the maximum diameter was found to be 33 μm inthe peripheral portion of the array and the distribution of diameters ofbottoms of the plated layers was found to be about 55%. It can be seentherefrom that a mold for forming a microlens array with a reduceddiameter distribution (reduced to 28% from 55%) can be fabricated inthis embodiment.

[0060] As described in the foregoing, in the mold of this embodiment,the distribution or variation of the semispherical or semicylindricalplated layers is decreased by the formation of the electroless platedlayer on the electroplated layer. Further, since the electroless platedlayer 6 contains phosphorus, corrosion resistance and wear resistance ofthe mold are improved as compared with those of a mold produced by usingonly electrodeposition or electroplating.

[0061] Any material, such as metal, semiconductor (a silicon wafer orthe like) and insulating substance (such as glass, quartz and polymerfilm), can be used as the substrate material. When the metal material isused as the substrate 1, there is no need to form the electrode layer 2.Further, when the semiconductor is used, the electrode layer is notnecessarily needed if the semiconductor has enough conductivity toenable electroplating. However, where metal or semiconductor is used asthe substrate, a plated layer will also be formed on a portion otherthan the microstructure forming portion, since the entire substrate isimmersed in the electroplating solution. Therefore, when the platedlayer is to be formed only on a predetermined portion, the insulatingsubstance can be preferably used as the substrate. Alternatively, ametal or semiconductor, whose surface is partially insulated, may alsobe used.

[0062] Materials of the electrode layer and the substrate are selectedfrom materials which are not corrosive to the electro- orelectroless-plating or electrodepositing solution used since theelectrode layer is exposed to such a liquid. The mask layer 3 may beformed of any inorganic or organic insulating material that is alsoanticorrosive to the electro- or electroless-plating orelectrodepositing solution. Further, it is preferable that it is harderto deposit the electroless plated layer on the material of the masklayer 3 than on the previously-formed plated or electrodeposited layerwhen electroless plating is conducted. Such a material is suitable forthe mask layer 3. The material of the mask layer 3 is also anticorrosiveto the electro- or electroless-plating or electrodepositing solution.

[0063] Where the electroplating or electrodeposition is effected at theopening 4 in the electroplating or electrodeposition solution 20containing ions such as metal ions, ions in the electroplating orelectrodeposition solution 20 move toward the plated layer 2, and hence,deposition of the electroplating or electrodeposition proceeds with itsgrowth direction being isotropic, as illustrated in FIG. 5. Thus, asemispherical or semicylindrical layer can be formed. When the size ofthe opening 4 is sufficiently smaller than the size of the anodic plate8 and ions are uniformly dissolved in the electroplating orelectrodeposition solution 20, the growth direction of the plated layeris isotropic. Typically, a microlens array has a structure of arrayedminute lenses each having a diameter from about 2 or 3 microns to about200 or 300 microns, and the size of the opening 4 is made smaller thanthe desired diameter of the microlens. In order to better achieve anisotropic growth of the plated or electrodeposited layer, the size ofthe opening is less than the diameter of the semispherical structure.

[0064] In the case of electroplating, the plated layer is formed by thedeposition of metal ions in the electroplating bath caused by theelectrochemical reaction. The thickness of the plated layer can bereadily controlled by controlling the electroplating time andtemperature. The following materials can be used as electroplatingmetal. For example, as a single metal, Ni, Au, Pt, Cr, Cu, Ag, Zn andthe like can be employed. As an alloy, Cu—Zn, Sn—Co, Ni—Fe, Zn—Ni andthe like can be used. Any material can be used so long as electroplatingis possible.

[0065] As the electrodeposition substance electrodeposited using acurrent and the base 7 in a conventional electrodeposition apparatus,there can be employed electrodepositable organic compound (acryl-seriescarboxylic acid resin and the like in the case of the anionic-typeelectrodeposition, and epoxy-series resin and the like in the case ofthe cationic-type electrodeposition).

[0066]FIG. 4 illustrates an electroless plating apparatus. Theelectroless plated layer 6 is grown on the semispherical orsemicylindrical plated or electrodeposited layer 5 until a desiredradius is reached. The diameter distribution of final microstructuresresults from the plated or electrodeposited layer, but not from theelectroless plated layer. The deposition mechanism of the electrolessplated layer is due to an oxidation-reduction reaction of metallic saltthat requires no current for deposition of the plated layer. Theelectroless plated layers 6 uniformly grow over the entire array.Electroless plating is stopped when the substrate is taken out ofelectroless plating liquid 30 and washed by water after the desiredprofile is achieved.

[0067] In the case of electroless plating, the thickness of the platedlayer 6 can be readily controlled by controlling electroless platingtime and temperature. The following materials can be used as electrolessplating metal, for example. As a single metal, Ni, Au, Cu, Co and thelike can be employed. As an alloy, Co—Fe, Co—W, Ni—Co, Ni—Fe, Ni—W andthe like can be used. Any material can be used so long as electrolessplating is possible.

[0068] As the reducing agent, sodium hypophosphite, potassiumhypophosphite, sodium borohydride, potassium borohydride, hydrazine,formalin, tartaric acid and the like can be employed. When sodiumhypophosphite or potassium hypophosphite is used as the reducing agent,the electroless plated layer contains phosphorus. Hence, corrosionresistance and wear resistance of the plated layer can be improved.

[0069] As discussed above, profiles of plated or electrodeposited layerscan be readily controlled by controlling processing time andtemperature, so the method has excellent controllability. When theelectroless plating after the initial electroplating orelectrodeposition is stopped immediately before the desired profile isreached and the plated layer of electroplating material with highcorrosion resistance and hardness is then grown until the desiredprofile is achieved, a microstructure array such as a mold for amicrolens array with high corrosion resistance and hardness can beobtained.

[0070] A process of forming a microlens array by using the abovestructure will be described. A resin of ultraviolet-ray hardeningphotopolymer is deposited on a mold for a microstructure array obtainedby the above fabrication method. After a support substrate of glass isplaced on the resin, the resin is hardened by exposing the resin toultraviolet rays. The resin of a microlens array can be separated fromthe substrate with the microstructure array by lifting the glasssubstrate. Thus, a concave resin of the microstructure array can beformed.

[0071] Another resin with a larger refractive index than that of theabove resin is further dropped on the concave resin, and the resin ishardened. Thus, a plane microlens array can be obtained.

[0072] In the above method, alkaline glass is not indispensable forforming a microlens, so that materials of the microlens and thesubstrate are less restricted than in the ion exchange method.

[0073] The above microlens array may be fabricated by other methods,such as a method in which a conventional thermoplastic resin is used anda heated mold is stamped on this resin; a method in which athermosetting resin is laid over a mold and then heated to be hardened;and a method in which an electron-beam hardening resin is coated on amold and the resin is hardened by electron beam irradiation.

[0074] A fabrication process of forming a mold for forming a microlensarray by using the above structure will be described.

[0075] This fabrication method may further include a step of forming amold on the the substrate obtained by the above fabrication method, anda step of separating the mold from the substrate. In this case, the moldmay be formed using electroplating. Then, a convex microlens array isfabricated by molding using the mold.

[0076] In this fabrication method, a plurality of molds with the sameprofile can be readily fabricated since the mold is formed by molding.In this embodiment, a plurality of molds for a microlens array with thesame profile can be fabricated using the same mold master.

[0077] After ultraviolet-ray hardening resin of photopolymer is laidover the mold for a convex microlens array fabricated by the abovemethod, a glass substrate as a supporting substrate is then placed onthe resin. The resin is exposed to ultraviolet rays through the glass tobe hardened. After that, the glass with the resin is separated from themold. Thus, a convex microlens array is obtained. A plurality ofmicrolens arrays of photopolymer could be formed by repeating the samesteps using the same mold.

[0078] The above microlens array may also be fabricated by othermethods, such as the above-described methods using thermoplastic resin,thermosetting resin and electron-beam hardening resin.

[0079] In this method, the mold can be directly formed by electroplatingor the like. Therefore, no expensive equipment is needed, costs can bereduced, and the size of the mold can be enlarged readily. Furthermore,the size of the plated layer can be controlled in situ, and the lensdiameter and the like can be readily and precisely controlled bycontrolling processing time and temperature.

[0080] (Second Embodiment)

[0081] A second embodiment of a fabrication method of a semisphericalmicrostructure array will be described by reference to FIGS. 2A to 2D asthe first embodiment is described.

[0082] Initially, electrode layer 2, mask layer 3 and openings 4 areformed on a substrate 1 as in the first embodiment.

[0083] Ni (nickel) electroplating is then performed for ten (10) secondsat a bath temperature of 50° C. and a cathodic current density of 40A/dm². The above substrate 1 for electroplating is used as a base 7, andthe electrode layer 2 is used as the cathode as illustrated in FIG. 3.Ni electroplating bath containing nickel (II) sulfate, nickel (II)chloride, boric acid and brightener is used. Thus, semispherical orsemicylindrical plated layers 5 are deposited until a diameter of itsbottom portion reaches 6 μm in a central portion of the array, asillustrated in FIG. 2C.

[0084] Ni electroless plating is then performed at a bath temperature of90° C. to form electroless plated layers 6 as illustrated in FIG. 2D. Nielectroless plating solution (S-780) containing a reducing agent ofhypophosphite is used.

[0085] The Ni plated layer 6 obtained by the electroless platingcontains phosphorus. When measured, a diameter of a bottom portion ofthe plated layers 5 and 6 in the central portion of the array was 21 μmand a diameter of a bottom portion of the plated layers 5 and 6 in aperipheral portion of the array was 23 μm (the maximum value). Thedistribution of diameters of bottoms of the plated layers in central andperipheral portions of the array was about 10%. The diameterdistribution of the second embodiment is smaller than that of the firstembodiment because a ratio of the electroplated layer 5 to the platedlayers 5 and 6 is reduced in the second embodiment.

[0086] Also in the mold of this embodiment, the distribution orvariation of the semispherical or semicylindrical plated layers isdecreased by the formation of the electroless plated layer on theelectroplated layer. Further, since the electroless plated layer 6contains phosphorus, corrosion resistance and wear resistance of themold are improved as compared with those of a mold produced by usingonly electrodeposition or electroplating.

[0087] (Third Embodiment)

[0088] A third embodiment of a method for fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 6A to 6E.

[0089] Initially, electrode layer 11, mask layer 12 and openings 13 areformed on a substrate 10 as illustrated in FIGS. 6A and 6B similarly tothe first embodiment.

[0090] A Cu (copper) electroplating is then performed for two (2)minutes at a bath temperature of 55° C. and a cathodic current densityof 4 A/dm² as illustrated in FIG. 3. The above substrate 10 forelectroplating is used as a base 7, and the electrode layer 11 is usedas the cathode. A Cu electroplating bath containing copper sulfate,sulfuric acid, hydrochloric acid and brightener is used. A Cu platedlayer 14 is initially deposited in the opening 13 and grown therein. Theplated layer 14 expands onto the mask insulating layer 12 as illustratedin FIG. 6C. The plated layer 14 is deposited until a diameter of itsbottom portion reaches 6 μm in a central portion of the array.

[0091] A Au (gold) electroless plating is then performed at a bathtemperature of 93° C. to form electroless plated layers 15 asillustrated in FIG. 6D. A Au (gold) electroless plating solutioncontaining potassium gold cyanide, ammonium chloride, sodium citrate andsodium hypophosphite is used.

[0092] The Au plated layers 15 obtained by the electroless platingcontains phosphorus. The electroless plated layer 15 is deposited untila diameter of its bottom portion reaches 15 μm in the central portion ofthe array.

[0093] A Cr (chromium) electroplating is then performed for sixty (60)seconds at a bath temperature of 50° C. and a cathodic current densityof 4 A/dm² to form a plated layer 16 and hence improve the corrosionresistance of the plated layer, as illustrated in FIG. 6E. The aboveelectroless plated layer 15 is used as the cathode. A Cr electroplatingbath containing chromic acid and sulfuric acid is used. Thus, the Cuplated layer 14 formed by electroplating, the Au electroless platedlayer 15 containing phosphorus, and the Cr plated layer 16 formed byelectroplating are deposited on the electrode layer 11 in this order.

[0094] When measured, a diameter of a bottom portion of the platedlayers 14, 15 and 16 in the central portion of the array was 20 μm and adiameter of a bottom portion of the plated layers 14, 15 and 16 in aperipheral portion of the array was 24 μm (the maximum value). Thedistribution of the diameters of the bottoms of the plated layers incentral and peripheral portions of the array was about 20%.

[0095] Also in the mold of this embodiment, the distribution orvariation of the semispherical or semicylindrical plated layers isdecreased by the formation of the electroless plated layer on theelectroplated layer. Further, since the chromium plated layer 16 isformed on the surface, hardness of the mold is improved.

[0096] (Fourth Embodiment)

[0097] A fourth embodiment of a method of fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 6A to 6E.

[0098] Initially, electrode layer 11, mask layer 12 and openings 13 areformed on a substrate 10 as illustrated in FIGS. 6A and 6B, similarly tothe third embodiment.

[0099] Ni electroplating is then performed for ten (10) seconds at abath temperature of 50° C. and a cathodic current density of 40 A/dm².The above substrate 10 for electroplating is used as a base 7, and theelectrode layer 11 is used as the cathode as illustrated in FIG. 3. Nielectroplating bath containing nickel (II) sulfate, nickel (II)chloride, boric acid and brightener is used. Thus, semispherical orsemicylindrical plated layers 14 are deposited until a diameter of itsbottom portion reaches 6 μm in a central portion of the array, asillustrated in FIG. 6C.

[0100] Ni electroless plating is then performed at a bath temperature of90° C. to form electroless plated layers 15 as illustrated in FIG. 6D.Ni electroless plating solution (S-780) containing a reducing agent ofhypophosphite is used. The Ni plated layers 15 obtained by theelectroless plating contain phosphorus. The plated layer 15 is depositeduntil a diameter of its bottom portion reaches 21 μm in a centralportion of the array.

[0101] A Au (gold) electroless plating is then performed for two (2)minutes at a bath temperature of 93° C. to form electroless platedlayers 16 and hence improve the corrosion resistance of the platedlayer, as illustrated in FIG. 6E. The above electroless plated layer 15is used as the cathode. A Au (gold) electroless plating solutioncontaining potassium gold cyanide, ammonium chloride, sodium citrate andsodium hypophosphite is used.

[0102] Thus, the Ni plated layer 14 formed by electroplating, the Nielectroless plated layer 15 containing phosphorus, and the Auelectroless plated layer 16 containing phosphorus are deposited on theelectrode layer 11 in this order.

[0103] When measured, a diameter of a bottom portion of the platedlayers 14, 15 and 16 in the central portion of the array was 21 μm and adiameter of a bottom portion of the plated layers 14, 15 and 16 in aperipheral portion of the array was 23 βm (the maximum value). Thedistribution of diameters of bottoms of the plated layers in central andperipheral portions of the array was about 10%.

[0104] Also in the mold of this embodiment, the distribution orvariation of the semispherical or semicylindrical plated layers isdecreased by the formation of the electroless plated layers on theelectroplated layer. Further, since the gold electroless plated layer 16containing phosphorus is formed on the surface, the corrosion resistanceof the surface of the mold is improved.

[0105] While the present invention has been described with respect towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. The present invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of fabricating an array ofmicrostructures comprising the steps of: preparing a substrate with anelectrically-conductive portion; forming an insulating mask layer on theelectrically-conductive portion; forming a plurality of openings in theinsulating mask layer to expose the electrically-conductive portion;forming a first plated or electrodeposited layer in the openings and onthe insulating mask layer by electroplating or electrodeposition; andforming a second plated layer on the first plated or electrodepositedlayer and on the electrically-conductive portion by electroless plating.2. A fabrication method according to claim 1, wherein the second platedlayer is formed by electroless plating using an electroless platingsolution containing a hypophosphite.
 3. A fabrication method accordingto claim 1, further comprising a step of forming a third plated layer onthe second plated layer by electroplating.
 4. A fabrication methodaccording to claim 1, further comprising a step of forming a thirdplated layer on the second plated layer by electroless plating.
 5. Afabrication method according to claim 4, wherein the third plated layeris formed by electroless plating using an electroless plating solutioncontaining a hypophosphite.
 6. A fabrication method according to claim1, wherein the opening has a circular shape and the microstructure is asemispherical microstructure.
 7. A fabrication method according to claim1, wherein the opening has an elongated stripe shape and themicrostructure is a semicylindrical microstructure.
 8. A fabricationmethod according to claim 1, wherein the first plated layer is formed byelectroplating.
 9. A fabrication method according to claim 1, whereinthe first electrodeposited layer is formed by electrodeposition using anelectrodepositable organic compound.
 10. A fabrication method accordingto claim 1, wherein microstructures of the array are continuously formedat their skirt portions.
 11. A fabrication method according to claim 1,wherein the first plated or electrodeposited layer and the second platedlayer are formed such that a bottom diameter or width of a semisphericalor semicylindrical microstructure is approximately in a range from 1 μmto 200 μm.
 12. A fabrication method according to claim 3, wherein thefirst plated or electrodeposited layer, the second plated layer and thethird plated layer are formed such that a bottom diameter or width of asemispherical or semicylindrical microstructure is approximately in arange from 1 μm to 200 μm.
 13. A fabrication method according to claim4, wherein the first plated or electrodeposited layer, the second platedlayer and the third plated layer are formed such that a bottom diameteror width of a semispherical or semicylindrical microstructure isapproximately in a range from 1 μm to 200 μm.
 14. A fabrication methodaccording to claim 1, wherein the first plated or electrodeposited layerand the second plated layer are formed such that a distribution ofbottom diameters or widths of semispherical or semicylindricalmicrostructures is approximately less than 20%.
 15. A fabrication methodaccording to claim 3, wherein the first plated or electrodepositedlayer, the second plated layer and the third plated layer are formedsuch that a distribution of bottom diameters or widths of semisphericalor semicylindrical microstructures is approximately less than 20%.
 16. Afabrication method according to claim 4, wherein the first plated orelectrodeposited layer, the second plated layer and the third platedlayer are formed such that a distribution of bottom diameters or widthsof semispherical or semicylindrical microstructures is approximatelyless than 20%.
 17. A fabrication method according to claim 1, whereinthe first plated or electrodeposited layer is formed such that a ratioof a diameter or width of the first plated or electrodeposited layerrelative to a diameter or width of a semispherical or semicylindricalmicrostructures is approximately less than 0.5.
 18. A fabrication methodaccording to claim 1, wherein the first plated or electrodeposited layeris formed such that a diameter or width of the first plated orelectrodeposited layer is approximately less than 10 μm.
 19. Afabrication method according to claim 1, further comprising a step offorming a mold on the substrate with the first plated orelectrodeposited layer and the second plated layer; and a step ofseparating the mold from the substrate.
 20. A fabrication methodaccording to claim 3, further comprising a step of forming a mold on thesubstrate with the first plated or electrodeposited layer, the secondplated layer and the third plated layer; and a step of separating themold from the substrate.
 21. A fabrication method according to claim 4,further comprising a step of forming a mold on the substrate with thefirst plated or electrodeposited layer, the second plated layer and thethird plated layer; and a step of separating the mold from thesubstrate.
 22. A fabrication method according to claim 1, furthercomprising a step of coating the substrate having the first plated orelectrodeposited layer and the second plated layer with a first resin; astep of hardening the first resin; a step of separating the first resinfrom the substrate; and a step of coating the hardened first resin witha second resin having a refractive index different from a refractiveindex of the first resin.
 23. A fabrication method according to claim 3,further comprising a step of coating the substrate having the firstplated or electrodeposited layer, the second plated layer and the thirdplated layer with a first resin; a step of hardening the first resin; astep of separating the first resin from the substrate; and a step ofcoating the hardened first resin with a second resin having a refractiveindex different from a refractive index of the first resin.
 24. Afabrication method according to claim 4, further comprising a step ofcoating the substrate having the first plated or electrodeposited layer,the second plated layer and the third plated layer with a first resin; astep of hardening the first resin; a step of separating the first resinfrom the substrate; and a step of coating the hardened first resin witha second resin having a refractive index different from a refractiveindex of the first resin.
 25. A fabrication method according to claim 1,wherein the array of microstructures is a microlens array.
 26. Amicrostructure array comprising: a substrate having anelectrically-conductive portion; an insulating mask layer formed on theelectrically-conductive portion, a plurality of openings being formed inthe insulating mask layer to expose the electrically-conductive portion;a first plated or electrodeposited layer formed in the openings and onthe insulating mask layer by electroplating or electrodeposition; and asecond plated layer formed on the first plated or electrodeposited layerand on the electrically-conductive portion by electroless plating.
 27. Amicrostructure array according to claim 26, wherein the second platedlayer contains phosphorus.
 28. A microstructure array according to claim26, further comprising a third plated layer formed on the second platedlayer by electroplating.
 29. A microstructure array according to claim26, further comprising a third plated layer formed on the second platedlayer by electroless plating.
 30. A microstructure array according toclaim 29, wherein the third plated layer contains phosphorus.
 31. Amicrostructure array according to claim 26, wherein the opening has acircular shape and the microstructure is a semispherical microstructure.32. A microstructure array according to claim 26, wherein the openinghas an elongated stripe shape and the microstructure is asemicylindrical microstructure.
 33. A microstructure array according toclaim 26, wherein the first plated layer is formed by electroplating.34. A microstructure array according to claim 26, wherein the firstelectrodeposited layer is formed by electrodeposition using anelectrodepositable organic compound.
 35. A microstructure arrayaccording to claim 26, wherein microstructures of the array arecontinuously formed at their skirt portions.
 36. A microstructure arrayaccording to claim 26, wherein the first plated or electrodepositedlayer and the second plated layer are formed such that a bottom diameteror width of a semispherical or semicylindrical microstructure isapproximately in a range from 1 μm to 200 μm.
 37. A microstructure arrayaccording to claim 28, wherein the first plated or electrodepositedlayer, the second plated layer and the third plated layer are formedsuch that a bottom diameter or width of a semispherical orsemicylindrical microstructure is approximately in a range from 1 μm to200 μm.
 38. A microstructure array according to claim 29, wherein thefirst plated or electrodeposited layer, the second plated layer and thethird plated layer are formed such that a bottom diameter or width of asemispherical or semicylindrical microstructure is approximately in arange from 1 μm to 200 μm.
 39. A microstructure array according to claim26, wherein the first plated or electrodeposited layer and the secondplated layer are formed such that a distribution of bottom diameters orwidths of semispherical or semicylindrical microstructures isapproximately less than 20%.
 40. A microstructure array according toclaim 28, wherein the first plated or electrodeposited layer, the secondplated layer and the third plated layer are formed such that adistribution of bottom diameters or widths of semispherical orsemicylindrical microstructures is approximately less than 20%.
 41. Amicrostructure array according to claim 29, wherein the first plated orelectrodeposited layer, the second plated layer and the third platedlayer are formed such that a distribution of bottom diameters or widthsof semispherical or semicylindrical microstructures is approximatelyless than 20%.
 42. A microstructure array according to claim 26, whereinthe first plated or electrodeposited layer is formed such that a ratioof a diameter or width of the first plated or electrodeposited layerrelative to a diameter or width of a semispherical or semicylindricalmicrostructures is approximately less than 0.5.
 43. A microstructurearray according to claim 26, wherein the first plated orelectrodeposited layer is formed such that a diameter or width of thefirst plated or electrodeposited layer is approximately less than 10 μm.44. A microstructure array according to claim 26, wherein themicrostructure array is a mold for forming a microlens array, alenticular lens or a flyeye lens.